Method for cutting a block of material and forming a thin film

ABSTRACT

A process for cutting out a block of material includes a step of introducing ions in the block thereby forming an embrittled zone and defining at least one superficial part of the block. The method also includes a step of forming at least one separation initiator at the level of the embrittled zone, wherein the step of forming the separation initiator includes implanting ions of an ionic nature different from that introduced during the preceding step. The method further includes a step of separating at the level of the embrittled zone the superficial part of the block from a remaining part of the block from the separation initiator, wherein the separation step includes at least one of a thermal treatment and the application of mechanical forces acting between the superficial part and the embrittled zone.

TECHNICAL FIELD

In a general way, the present invention relates to a process for cuttingout a block of material. This process can be implemented, in particular,for the formation of thin films.

Thin films, self-supporting or integral with a support substrate, arewidely used in the fields of micro-electronics, opto-electronics andmicro-mechanics. Thus the invention is applicable to these domains, inparticular for producing components or integrated circuits.

STATE OF PRIOR ART

As mentioned above, the use of thin films is increasingly widespread forcomponents whose operation or manufacturing method requires specialphysical and electrical properties.

Thin films have a thickness which is usually comprised between a fewnanometers and a few micrometers. They also make it possible, forexample, to use materials whose usage under the form of a thicksubstrate would be ruled out for reasons of cost or compatibility withother materials used.

The compatibility of materials can also constitute an obstacle to thedirect formation of a thin film on a support substrate on which it isfinally used. A certain number of processes have been developed to formfirst of all a thin film on a source substrate and then to transfer thethin film from the source substrate to a target substrate.

These processes as well as other techniques concerning the manufactureand transfer of thin films are described in documents [1], [2], [3],[4], [5], [6] and [7], whose complete references are given at the end ofthe present description.

In particular, document [1] describes the possibility of forming anembrittled zone in a plate of material by ionic implantation in order todetach the superficial thin film from the plate later at the level ofthis zone.

The separation of the thin film from the source substrate is provoked,or at least assisted, by using a certain number of mechanical or thermalstresses. In particular, the cutting out of the thin film requires anenergy budget under thermal and/or mechanical form, which is linkedmainly to the dose of the species implanted to form the embrittled zone.

The implementation of techniques for cutting out and transferring a thinfilm, as described in the documents quoted above, can be linked to acertain number of difficulties. For example, the use of certainmaterials with a high thermal expansion coefficient is not compatiblewith a thermal treatment at too high a temperature. For certainsubstrates it is also necessary to limit the dose of the speciesimplanted either to preserve the thin film or for economical reasons.

Furthermore, the implementation of mechanical forces to separate thesource substrate from the thin film, such as described above inreference [7], also makes it possible to reduce the thermal budget forfracture, especially in the case where the materials in contact havedifferent expansion coefficients. The application of mechanical effortson the source substrate and/or the target support is however not alwayspossible, especially when the materials being used are brittle, or whenthe cleavage zone has not been sufficiently embrittled by ionicimplantation.

Finally, the techniques of separation and transfer of the thin film,described above, involve a certain number of restrictions andcompromises. These restrictions are imposed, in particular, by the typeof materials used to constitute the source substrate, the thin film andthe target support.

DESCRIPTION OF THE INVENTION

The aim of the invention is to propose a cutting out method making itpossible, in particular, to form and transfer thin films, without thelimitations mentioned above.

A further aim of the invention is to propose a cutting out method ableto be implemented with a reduced energy budget and in particular areduced thermal budget.

Another aim of the invention is to propose an economical process inwhich an eventual implantation of impurities, intended to form anembrittled zone, can be carried out with a reduced dose.

In order to attain these aims, the invention has more precisely the aimof a method for cutting out a block of material, comprising thefollowing stages:

(a) the formation in the block of a buried zone, embrittled by at leastone stage of ion introduction, the buried zone defining at least onesuperficial part of the block,

(b) the formation at the level of the embrittled zone of at least oneseparation initiator by the use of a first means of separation chosenfrom amongst the insertion of a tool, the injection of a fluid, athermal treatment and/or implantation of ions of an ionic naturedifferent from that introduced during the preceding stage, and

(c) separation at the level of the embrittled zone of the superficialpart of the block from a remaining part, called the mass part, from theseparation initiator by the use of a second means, different from thefirst means of separation and chosen from among a thermal treatmentand/or the application of mechanical forces acting between thesuperficial part and the embrittled zone.

The separation initiator(s) can be located on all or part of theperiphery of the block and/or on local internal zones of the block, andare capable of spreading into the embrittled zone.

The invention is based on the fact that it is possible to reducesignificantly the overall energy for providing the block (either ofthermal origin and/or mechanical origin) for implementing a cutting outprocess, by forming a separation initiator before the actual separation.

The mechanical stresses which can be used profitably for the separationcan be stresses applied from outside the block or internal stressespresent in the block.

Although the stages are preferably carried out successively in the orderindicated, it is possible, at least for certain applications, to carryout stages a and b concomitantly. Moreover, stages b and c can also beconcomitant.

According to a special implementation of the process, intended for themanufacture of thin films, it is possible to form an embrittled zoneextending closely parallel to a closely plane face of the block, todefine in the block a superficial part in the form of a thin superficialfilm.

By closely plane face, it is understood a face whose mean plane is flat,but which can comprise surface micro-rugosities whose rugosity valuesrange from a few tenths of nanometers to several hundred nanometers. Theinventors have been able to show that an implantation across a surfacewith a micro-rugosity, for example with an RMS (root mean square height)value of 10 nm, does not disturb the embrittling mechanism andsubsequent fracture. This fact is interesting since this rugosity is ofthe order of the rugosity of the free face of the film after transfer.Therefore in these conditions, it is possible to recycle the samesubstrate several times without recourse to surface polishing.

Advantageously the embrittled buried zone can be formed by implantation.

It involves, for example, an implantation of gaseous species enablingthe formation of a thin film of microcavities in the block of material.This film defines the superficial part to be cut out and embrittles theblock of material locally.

By gaseous species one understands elements, such as hydrogen or raregases, for example, in their atomic form (for example H), in theirmolecular form (for example H₂), in their ionic form (for example H⁺, H₂⁺), in their isotopic form (for example deuterium), or in their isotopicand ionic form.

Moreover, by implantation one means any technique for introducing thespecies mentioned above into the block, such as bombardment, diffusionetc. These techniques can be implemented separately or in combination ofseveral together.

As an illustration of implantation techniques, one can refer to thedocuments quoted above. Nonetheless, thanks to the formation of aseparation initiator, according to the invention, the doses of thespecies implanted to form the embrittled zone can be reduced. Thereduced doses make it possible to lower the disturbance of the state ofthe surface of the thin films, or parts cut out, and thus to control therugosity.

According to a particular embodiment of the invention, an implantationcan be carried out locally by overdosing to form the separationinitiator, the first means of separation then corresponding to anoverdosing.

This possibility is very interesting in so far as a high doseimplantation only takes place in a small part of the block of material.Besides, as indicated above, a much lower dose can be used to form theembrittled zone.

The separation initiator can be formed in a same plane as the embrittledzone as a prolongation of this zone. If the initiation of the initiatoris carried out in another plane than that of the embrittled zone, thepropagation of the initiator rejoins the embrittled zone.

Several possibilities can be retained for forming the separationinitiator.

According to a first possibility, the separation initiator can be formedby an ionic implantation of a species which is different from thatreserved for the formation of the embrittled zone.

According to another possibility, one can form the separation initiatorby inserting a tool in the block. The first means of separation thencorresponds to the insertion of the tool.

According to another possibility, the separation initiator can be formedby local injection of a fluid on the block. The first means ofseparation then corresponds to the injection of fluid.

According to a further possibility the separation initiator can beformed by local thermal treatment of the block. The first means ofseparation then corresponds to the local thermal treatment.

In an application of the method of the invention to the formation of athin film, depending on its thickness, it can advantageously be madeintegral with a stiffener before the separation stage c (or even beforestage b). The stiffener can be deposited at the surface of the block ofmaterial, in contact with the thin film to be cut out, according to anydeposit technique. It can also be made integral with the thin film bymolecular adhesion or by gluing by means of a binder (glue).

On the other hand, when the thin film or the part to be cut out issufficiently thick or is in a material sufficiently rigid not to tear,the presence of a stiffener is not indispensable. In the remainder ofthe text, a part or a layer with a thickness or a rigidity sufficientnot to tear at the moment of separation, will be called a“self-supporting” part or layer.

Other characteristics and advantages of the invention will become clearfrom the following description, referring to the figures in the attacheddrawings. This description is given for purely illustrative andnon-limiting reasons.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A to 1D are diagrammatic cross-sections of a substrate andillustrate the stages for cutting out a thin film, supported by astiffener, according to a method conforming to the invention.

FIGS. 2A to 2C are diagrammatic cross-sections of a substrate andillustrate the stages for cutting out a self-supporting thin film,according to a method conforming to the invention.

FIGS. 3A to 3D are diagrammatic cross-sections of a substrate andillustrate the stages for cutting out a thin film, supported by astiffener, according to a method conforming to the invention andconstituting a variant relative to the process illustrated by FIGS. 1Ato 1D.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following description, identical, similar or equivalent parts inthe various figures are marked by the same references numbers, so as tomake it easier to refer from one embodiment to another.

Moreover, it should be pointed out that the different figures anddifferent parts of figures are not represented on a homogeneous scale,in order to increase the readability of the figures.

FIG. 1A shows a substrate 10 which constitutes a block of material,homogeneous or not, such as mentioned in the preceding description. Thisblock can, for example, be an ingot or a plate of semiconducting orpiezo-electric or ferro-electric material. It may be pre-treated or not.In the case where the block is a semiconducting plate, treated or not,it can, for example, be a silicon substrate.

Ionic implantation of hydrogen with a dose of the order of 7.10¹⁶H⁺/cm²at an energy of 100 keV, for example, makes it possible to form anembrittled zone 12 in the substrate. This extends closely along a planeparallel to the surface of the substrate through which the impuritieshave been implanted. In the example shown in the figure the impuritiesare implanted through a face of the substrate 18 which, in the textbelow, is called the superficial face. In the substrate 10 theembrittled zone defines a superficial thin film 14 and a mass part 16.

FIG. 1B shows the mounting of the superficial face of the thin film 14on a second substrate 20 called the target substrate and which canconstitute a stiffener for the thin film. It can, for example, be asubstrate of fused silica, commonly called quartz.

Solidarisation of the layer 14 with the substrate 20 can be carried outeither directly by molecular adhesion, or as shown in the FIG. 1 by theintermediary of at least one layer of material 22 set on the thin filmand/or on the substrate. In this latter case, the intermediary layer 22is chosen either to encourage the molecular adhesion (for example SiO₂)or to produce an adhesive gluing (for example a layer of glue).

In the case of direct molecular adhesion between the faces of the twosubstrates to be assembled, the substrates undergo, for example, achemical cleaning treatment intended to make the faces to be assembledhydrophilic. After putting the faces to be assembled into contact, thesubstrates can possibly undergo a first thermal treatment intended tostrengthen the adhesion forces. This treatment is carried out, forexample, with a thermal budget of the order of 300° C. for 2 hours.

FIG. 1C shows the formation of a separation initiator 30 in thesubstrate 10. The separation initiator 30 extends from the outside face32 of the substrate 10, in this case a lateral face on the figure, up tothe embrittled zone 12. The separation initiator can be provoked bydifferent means, represented symbolically on the figure under the formof an arrow with reference 34. These means can comprise injection ofwater or another fluid, or a tool such as a blade, inserted at the levelof the embrittled zone.

According to another possibility, the separation initiator can beprovoked by ionic implantation with an overdose limited to a region atthe edge of the substrate. Such a region is shown on the figures withthe reference 36.

Evidently, the overdose can be made in other regions of the substratesuch as, for example, a central region.

In this case, the formation of the separation initiator can possiblytake place during a same implantation stage implemented also for formingthe embrittled zone. To refer to the numeric example given above, region36 is over-implanted, for example with a dose of 9.10¹⁶H⁺/cm².

According to a further possibility, a separation initiator can beprovoked by overheating the substrate locally (for example with the aidof a laser or a local heat source).

It should be noted here that the term “separation initiator” means,within the framework of the present description, either a region inwhich the separation has already been started, or a region, particularlybrittle, in which the separation will be started at a later stage ofactual separation.

An arrow 34 a with dotted lines indicates the possibility of forming aplurality of separation initiators.

FIG. 1D shows a final stage of separation of the thin film 14 and themass part 16 of the substrate. The separation can be assisted byapplying mechanical forces, under the form of pressure, traction forcesof shearing or peeling, and/or a thermal treatment. As an example, inthe conditions mentioned above, one can carry out a thermal treatment ofa few minutes at 350° C. to obtain total separation. The thermal budgetused to obtain separation of the parts takes into account precedingthermal treatments which have been carried out such as, for example,thermal treatment to increase the adherence between the substrates. Inevery case, this thermal budget is reduced due to the use of theseparation initiator.

Finally, one obtains a structure formed of the target substrate 20 withthe thin film 14 at its surface.

The mass part 16 of the first substrate can be re-used for latercut-outs of another thin film. Possibly, it can also act as targetsubstrate for the support of another thin film of another material.

With the process illustrated in FIGS. 1A to 1D, it is possible, forexample, to obtain other structures as well, comprisingnon-semiconductor materials on a silicon substrate, for example, such asLiNbO₃, LiTaO₃ or SrTiO₃. It is also possible to transfer layers ofsemiconductor materials III-V on silicon or on other III-Vsemiconductors. The process can also be implemented to obtain substratesof the SOI type (silicon on insulator).

The following is an example of the process parameters which can beretained for the manufacture of an SOI support/P13.

At the time of the first stage one carries out an ionic implantation ofhydrogen with a dose of 7.10¹⁶H⁺/cm² at 100 keV in a standard plate ofsurface oxidised silicon. This implantation makes it possible to definea thin film limited by an embrittled zone. A local overdosing at9.10¹⁶H⁺/cm² is carried out at the periphery of the embrittled zone. Theoverdosing makes it possible to form a separation initiator in themeaning of the invention along a length of 1 to 2 cm, from the edge ofthe plate in the case of an initiator at the edge of the plate. Aftermounting the plate on another silicon plate on which the oxide layer isadhered, one proceeds with a thermal separation treatment. It is seenthat a treatment of 4 hours at 350° C. makes it possible to obtain aseparation which spreads from the initiator to the whole of theembrittled zone.

In the absence of the separation initiator, it should also be possibleto provoke separation. Nonetheless in this case, thermal treatment at350° C. for 11 hours would be required. This demonstrates thesignificant reduction in the thermal budget imposed on the substrates,resulting from the invention.

FIG. 2A, which shows the first stage of a second possibility ofimplementation of the invention, is identical to FIG. 1A. Therefore onecan refer to the description above concerning this figure.

FIG. 2B illustrates the formation of a separation initiator 30. It canbe seen that the initiator 30 is used close to the level of theembrittled zone 12 and that, moreover, the surface 18 of the thin film14 is left free.

FIG. 2C shows the final stage of separation which is provoked withoutequipping the thin film 14 with a stiffener. Such an implementation ofthe process is adapted in particular to the formation of self-supportingthin films.

FIGS. 3A to 3D show another possibility of implementing the process.FIGS. 3A and 3B are identical to FIGS. 1A and 1B, and therefore they donot have to described again.

FIG. 3C, which illustrates the formation of a separation initiator,shows that the means of separation 34 can be applied elsewhere than atthe level of the embrittled zone 12. In the example of FIG. 3C, a tool,such as a blade, is inserted on a lateral side 32 of the structure, atthe level of the interface between the first substrate 10 and the targetsubstrate 20. For example, the tool is inserted at the level of theintermediary layer 22 when this exists. Because of the relatively finethickness of the thin film, for example lower than or of the order ofseveral μm, that is to say of the low depth of the zone embrittled inthe first substrate 20, the separation initiator spreads through thethin layer to rejoin the embrittled zone 12.

FIG. D illustrates the final separation which spreads from the initiator30 across the whole of the surface of the thin film following theembrittled zone.

As mentioned above, the presence of a separation initiator makes itpossible to reduce the thermal budget of the final stage and/or reducethe implantation dose of the embrittled zone. By playing on these twoparameters, it is thus possible to control the rugosity of the mass part16 and, above all, the thin film 14.

DOCUMENTS QUOTED

-   [1] FR-A-2681472/U.S. Pat. No. 5,374,564-   [2] FR-A-2773261-   [3] FR-A-2748851-   [4] FR-A-9909007-   [5] U.S. Pat. No. 5,994,207-   [6] EP-A-0925888-   [7] FR-A-2748851

1. A process for cutting out a block of material, comprising:introducing ions in said block thereby forming an embrittled zone anddefining at least one superficial part of the block, performing athermal treatment to increase an embrittlement of the embrittled zoneafter said introducing, forming at least one separation initiator at alevel of the embrittled zone after said performing, by implanting ionsof an ionic species different from that introduced during saidintroducing, placing the at least one superficial part of the block incontact with a stiffener, and separating, at the level of the embrittledzone, the superficial part of the block from a remaining part of theblock, from the separation initiator, wherein said separating comprisesat least one of a thermal treatment and an application of mechanicalforces acting between the superficial part and the embrittled zone.
 2. Aprocess according to claim 1, wherein the separation initiator is formedover all or part of the periphery of the block.
 3. A process accordingto claim 1, wherein the embrittled zone extends closely parallel to aclosely plane face of the block, to define in the block a superficialthin film.
 4. A process according to claim 1, wherein the forming saidat least one separation initiator and the separating are concomitant. 5.A process according to claim 1, wherein said forming said at least oneseparation initiator comprises a local implantation with an overdose. 6.A process according to claim 1, wherein said separating comprisesapplying mechanical forces applied from outside the block.
 7. A processaccording to claim 3, further comprising: depositing at least one layeron the superficial thin film of a material forming said stiffener.
 8. Aprocess according to claim 3, wherein the superficial thin film is madeintegral with said stiffener.
 9. A process according to claim 1, furthercomprising: re-using the remaining part of the block of material bycutting out a new superficial part after said separating.
 10. A processaccording to claim 1, further comprising: re-using the remaining part ofthe block of material as a stiffener for a superficial part of anotherblock after said separating.
 11. A process according to claim 1, whereinthe separation initiator is formed on local internal zones of the block.12. A process according to claim 3, further comprising: gluing togetherthe superficial thin film and the stiffener.
 13. A process according toclaim 3, further comprising: coupling together the superficial thin filmand the stiffener via molecular contact adhesion.